Microearthquakes in and near Long Valley, California

Abstract

Sixteen portable seismograph stations were deployed in the vicinity of the Long Valley geothermal area, California, from April 27 to Ju, ne 2, 1973. Only minor microearthquake activity was detected in the Long Valley caldera, but a high level of activity was detected to the south and east of the caldera. The abrupt spatial seismicity decre. ase at the southern boundary of the caldera suggests that the caldera is either strucrurally less competent than the surrounding crust or is at a junction of different regional tectonic deformation trends. No significant attenuation or delays occurred for either local P or S waves that traversed the caldera.

Observation of shear wave splitting from natural events: South Moat of Long Valley Caldera, California, June 29 to August 12, 1982

“…The local stress field of Long Valley has mainly been studied through focal-mechanism analyses (Table 2, and Figure 16) [Pitt and Steeples, 1975;Steeples and Pitt, 1976;Cramer and Toppozada, 1980;Savage and Cockerham, 1984;Vetter, [Pitt and Steeples, 1975;Steeples and Pitt, 1976] show dominantly strike-slip mechanisms, all with tension axes generally ENE-WSW for events with focal depths of less than 10 km. Some normal-fault mechanisms were also found when the events were deeper than 10 km, especially inside the Sierra Nevada [Pitt and Steeples, 1975] .camp _en.t ........................ • _compo.…”

The Geophysics of a Restless Caldera Long Valley, California

“…Following this event, aftershock activity (up to ML � 4) gradually spread to the northwest toward the caldera over the next year and a half. Then, on May 25-27, 1980, a sequence of four ML = 6 events occurred, initiating activity within Long Valley caldera, which (based on limited data) had been regarded as seismically quiescent (Steeples & Pitt 1976, Van Wormer & Ryall 1980. The epicenters of the f i rst the event on May 27 (Nos.…”

Low-velocity zone under long valley as determined from teleseismic events

“…This indicates that if true magma is present along the ray paths in question, it is in small pockets (probably less than 4 km thick, the approximate wavelength of the S waves recorded), or it has sufficient viscosity to transmit S waves. Local earthquake S waves did not penetrate deep enough to pass through the heart of the anomalous body in the caldera [Steeples and Pitt, 1976, this issue].…”